1. Field of the Invention
The present invention relates generally to electronic packing technology, and more particularly, to a partial glob-top encapsulation technique using different encapsulants for encapsulating wire bonds of an electronic device, and a method thereof.
2. Description of the Prior Art
A so-called glob-top encapsulation is a conformal coating electronic packing technology, which can be used to protect bare die and wire bonds in a chip-on-board (COB) assembly. In the COB assembly technology, a bare die is directly mounted on a circuit board using epoxies for attaching, and the bare die is electrically connected to contact pads on the circuit board using wire bonding technology. The glob-top encapsulation technology forms an encapsulant by curing an adhesive material (e.g., epoxy or silicone) applied to the bare die, metal wires, and bonds, thereby providing a shell to protect them from stress related failures, contaminants, thermal shocks, and moisture.
In some optics-related application, such as a Liquid Crystal on Silicon (LCoS) device, there is a cover glass disposed above a bare die (or a silicon backplane). An encapsulant only covers some area of the die, but does not cover the whole cover glass, which is called partial glob-top encapsulation. In this technology, sometimes faces of the cover glass and the die are not aligned, which may form a notch around the corner of the die, thereby introducing a notch effect having a bad influence on fracture toughness of the encapsulant.
Please refer to FIG. 1, which illustrates a conventional partial glob-top encapsulation technology. Die 110 is electrically connected to a circuit board 120 through metal wires 130, and there is a cover glass 140 disposed above the die 110. Encapsulant 150 formed by curing an adhesive material covers the metal wires 130 and bonds 160. Generally, it is expected that the encapsulant 150 is rigid enough to resist external stresses. However, as a face 142 of the cover glass 140 is not aligned with a face 112 of the die 110, a notch 170 is therefore formed around the corner of the die 110. As a result, a notch effect is introduced. Such effect notch will cause the encapsulant 150 to become brittle because the stresses will concentrate at the notch 170, thereby causing fatigue cracks of the encapsulant 150. This effect will become stronger when the encapsulant 150 is more rigid.
A simplest solution to this issue is using a softer adhesive material. However, if the encapsulant 150 is formed by softer adhesive materials, the encapsulant 150 may become too weak to resist the external stresses such that the metal wires 130 and the bonds 160 may have poor endurance to physical damages. In view of this, there is a need to provide a different solution to address the above-mentioned issue.